The rare earth based permanent magnet with a R-T-B (R represents one or more elements selected from the group consisting of Y and rare earth elements, T represents one or more metal elements containing Fe or the combination of Fe and Co, and B represents B or the combination of B and C) based composition is well known as a permanent magnet which has R2T14B as the main phase, includes structure containing the grain boundary phase and has excellent magnetic properties such as a high energy product BHmax and the like, wherein the grain boundary phase contains a R-rich phase which contains more amount of R than the main phase. The R-T-B based rare earth based permanent magnet is used as a permanent magnet with good performances in motors which particularly require good performances, such as a voice-coil motor for head driving in hard disk drive or a motor in a electric vehicle, a hybrid car or the like.
Since the rare earth based permanent magnet contains a high amount of R which is highly active and R corrodes easily and has a bad corrosion resistance, the corrosion resistance can be improved by plating Ni on the surface of the rare earth based magnet to inhibit the corrosion starting from the surface.
With respect to the improvement of the corrosion resistance of the rare earth based permanent magnet body, it has been attempted to improve the corrosion resistance of the magnet body by adding elements such as Co or Cu or the like.
In the prior art, a rare earth based sintered magnet has been provided that an intermediate phase with good oxidation resistance and containing 30 atom % to 60 atom % of Co and Cu in total is set up by surrounding the R-rich phase which is present in the grain boundary triple point surrounded by three or more main phases. And thus, the oxidation of R components in the R-rich phase can be inhibited at the grain boundary triple point and the corrosion resistance can be improved accordingly (for example, see Patent Document 1).